In recent years, there have been widely prevailed storage systems each of which uses a plurality of large-capacity storage devices such as hard disk drives (HDDs) or the like. Usually, the storage system includes a plurality of storage devices and a control device controlling access to these storage devices. Individual devices in such a storage system are connected to one another through switches, for example. Connection is established between a switch and a switch or between a switch and another device, using a transmission path such as a communication cable, a printed-circuit board, or the like. In the case of the storage system, in many cases, a transmission path compliant with a standard such as Fibre Channel (FC), Serial Attached SCSI (SAS, SCSI: Small Computer System Interface), or the like is used.
For example, a control device or a storage device within the storage system is realized as a rack-mountable module device. For example, in some case, a switch for connecting devices to each other is realized as a single module device, and, in other cases, the switch is provided within a module device such as a control device, a storage device, or the like. In addition, in a storage system where the number of storage devices is large, storage devices and switches are contained in a plurality of racks, and racks are connected to one another using the above-mentioned transmission path, in some cases.
In addition, as an example of a system in which data is transmitted through a plurality of transmission paths, there is a system that includes a transmission-path multiplexing device transmitting data to the plural transmission paths through a bus subjected to time-division multiplexing and prevents a time-division channel number corresponding to a transmission path from being generated when abnormity occurs in the transmission path. An example of such a system is disclosed in Japanese Laid-open Patent Publication No. 60-196037.
In addition, as an example of a technique achieving the appropriate characteristic of a transmission path, there is an imaging device in which the transmission timing of an image synchronization signal is controlled on the basis of the detection result of a cable length. An example of such a technique is disclosed in Japanese Laid-open Patent Publication No. 2010-93543. Furthermore, there is also a skew adjustment method in which, on the basis of a measurement value of the signal propagation time of a pair of cables, an acceptable cable length is calculated and a cable length is adjusted, thereby reducing the skew of a cable. An example of such a technique is disclosed in Japanese Laid-open Patent Publication No. 2001-338541.
Incidentally, in a case in which high-speed serial transmission ranging from several hundred Mbps to several Gbps is performed in a transmission path, when a signal has passed through the transmission path, an insertion loss occurs in some cases. For example, this insertion loss occurs as a resistive loss due to a skin effect, a dielectric loss due to the dielectric tangent (tan δ) of substrate material, or the like. The insertion loss increases with an increase in a transmission frequency or with an increase in the transmission distance of a signal.
In the above-mentioned storage system, when a plurality of devices exist in the storage system, for example, when storage devices are contained in a plurality of racks, the length of a transmission path between switches becomes large, in some cases. When the length of the transmission path becomes large, there occurs a problem that the insertion loss increases and it is difficult to correctly transmit data. In particular, in the storage system, a data transmission speed between devices is requested to be enhanced, and when the data transmission speed is enhanced, the insertion loss further increases.